The invention relates to a dual-band planar antenna structure applicable in mobile communication devices, for example.
Mobile communication devices, especially those operating at two frequency bands, have grown more popular in recent years, subsequent to the introduction of frequency ranges around the two-gigahertz region. The lower frequency band is usually 890-960 MHz used by the GSM (Global System for Mobile telecommunications) system or 824-894 MHz used by the American AMPS (Advanced Mobile Phone System) network. The upper operating frequency band may be e.g. 1710-1880 MHz used by the DCS (Digital Cellular System) and PCN (Personal Communication Network) or 1850-1990 MHz used by the PCS (Personal Communication System). The future UMTS (Universal Mobile Telecommunication System) has been allocated transmission and reception bands in the 1900-2170 MHz range. Thus it is obvious that the operating bands may be relatively wide, which sets additional requirements on the antenna of a mobile communication device.
From the prior art it is known a number of antenna structures that have at least two operating frequency bands. Mobile communication devices use various combination antennas such as a combination of a whip and helix antenna or a combination of a whip and planar inverted-F antenna (PIFA). In addition, PIFA-type antennas are known which by themselves operate at two frequency ranges. FIG. 1 shows one such prior-art antenna structure. It comprises a radiating plane 110, a ground plane 120 parallel to said radiating plane, and a short-circuit element 102 between these two planes. In this example, the antenna is fed at a position 101 of its edge. The radiating plane 110 has a relatively narrow slot 115 in it, starting at one edge of the plane, making a rectangular bend, and extending close to the feed position 101. Viewed from the feed position, the slot 115 divides the plane 110 up into two branches 111 and 112. Operation at two frequency bands is based on the fact that these branches have quite different resonance frequencies. Antenna matching can be adjusted by varying the feed position 101 as well as the location of the short circuit 102. Desired values for the resonance frequencies of the antenna can be obtained by varying the location of the slot 115 and the number of bends in it. The disadvantage of the structure is that it may be difficult to accomplish a sufficient bandwidth at both operating frequency ranges. The frequency bands can be widened by increasing the distance between the radiating element and ground plane, but this arrangement has the drawback of making the antenna larger.
The primary object of the invention is to improve the band characteristics of a dual-band PIFA. The structure according to the invention is characterized by what is expressed in the independent claim 1. Preferred embodiments of the invention are presented in the other claims.
Described briefly, the invention is as follows: In the radiating element of the PIFA there is provided a slot consisting of two portions having different widths. One end of the wider portion of the slot is close to the feed point of the radiating element. The narrower portion of the slot begins at a point in the wider portion and extends to the edge of the radiating element. The portions of the slot are advantageously straight, but the narrower portion may have bends in it in order to form the branches of the radiating element. The ratio of the widths of the portions of the slot is order of three.
An advantage of the invention is that the bandwidths of a dual-band PIFA can be made larger than those of prior-art structures of the same size. Another advantage of the invention is that the structure according to it is simple and has relatively low manufacturing costs.
The invention will now be described in detail. Reference will be made to the accompanying drawing wherein
FIG. 1 shows an example of a PIFA according to the prior art,
FIG. 2 shows an example of a PIFA according to the invention,
FIG. 3a shows an example of the effect on the antenna characteristics of the narrower portion of the slot,
FIG. 3b shows an example of the effect on the antenna bandwidths of the ratio of the widths of the portions of the slot,
FIGS. 4a-i show alternative radiating element shapes according to the invention, and
FIG. 5 shows an example of a mobile communication device equipped with an antenna according to the invention.